In general, a compressor is a mechanical apparatus for compressing the air, refrigerant or other various operation gases and raising a pressure thereof, by receiving power from a power generation apparatus such as an electric motor or turbine. The compressor has been widely used for an electric home appliance such as a refrigerator and an air conditioner, or in the whole industry.
The compressors are roughly classified into a reciprocating compressor in which a compression space for sucking or discharging an operation gas is formed between a piston and a cylinder, and the piston is linearly reciprocated inside the cylinder, for compressing a refrigerant, a rotary compressor in which a compression space for sucking or discharging an operation gas is formed between an eccentrically-rotated roller and a cylinder, and the roller is eccentrically rotated along the inner wall of the cylinder, for compressing a refrigerant, and a scroll compressor in which a compression space for sucking or discharging an operation gas is formed between an orbiting scroll and a fixed scroll, and the orbiting scroll is rotated along the fixed scroll, for compressing a refrigerant.
Recently, a linear compressor which can improve compression efficiency and simplify the whole stricture without a mechanical loss resulting from motion conversion by connecting a piston directly to a linearly-reciprocated driving motor has been popularly developed among the reciprocating compressors.
FIG. 1 is a side cross sectional view showing a conventional reciprocating compressor. In the reciprocating compressor, an inlet tube 2a and an outlet tube 2b through which refrigerants are sucked and discharged are installed at one side of a closed vessel 2, a cylinder 4 is fixedly installed inside the closed vessel 2, a piston 6 is installed inside the cylinder 4 to be linearly reciprocated to compress the refrigerants sucked into a compression space P in the cylinder 4, and various springs are installed to be elastically supported in the motion direction of the piston 6. Here, the piston 6 is connected to a linear motor 10 for generating a linear reciprocation driving force.
In addition, a suction valve 22 is installed at one end of the piston 6 contacting the compression space P, and a discharge valve assembly 24 is installed at one end of the cylinder 4 contacting the compression space P. The suction valve 22 and the discharge valve assembly 24 are automatically controlled to be opened or closed according to the inside pressure of the compression space P, respectively.
The top and bottom shells of the closed vessel 2 are coupled to hermetically seal the closed vessel 2. The inlet tube 2a through which the refrigerants are sucked and the outlet tube 2b through which the refrigerants are discharged are installed at one side of the closed vessel 2. The piston 6 is installed inside the cylinder 4 to be elastically supported in the motion direction to perform the linear reciprocation. The linear motor 10 is connected to a frame 18 outside the cylinder 4. The cylinder 4, the piston 6 and the linear motor 10 compose an assembly. The assembly is installed on the inside bottom surface of the closed vessel 2 to be elastically supported by a support spring 29.
The inside bottom surface of the closed vessel 2 contains oil, an oil supply device 30 for pumping the oil is installed at the lower end of the assembly, and an oil supply tube 18a for supplying the oil between the piston 6 and the cylinder 4 is formed inside the frame 18 at the lower side of the assembly. Accordingly, the oil supply device 30 is operated by vibrations generated by the linear reciprocation of the piston 6, for pumping the oil, and the oil is supplied to the gap between the piston 6 and the cylinder 4 along the oil supply tube 18a, for cooling and lubrication.
The cylinder 4 is formed in a hollow shape so that the piston 6 can perform the linear reciprocation, and has the compression space P at its one side. Preferably, the cylinder 4 is installed on the same straight line with the inlet tube 2a in a state where one end of the cylinder 4 is adjacent to the inside portion of the inlet tube 2a. 
The piston 6 is installed inside one end of the cylinder 4 adjacent to the inlet tube 2a to perform linear reciprocation, and the discharge valve assembly 24 is installed at one end of the cylinder 4 in the opposite direction to the inlet tube 2a. 
Here, the discharge valve assembly 24 includes a discharge cover 24a for forming a predetermined discharge space at one end of the cylinder 4, a discharge valve 24b for opening or closing one end of the cylinder 4 near the compression space P, and a valve spring 24c which is a kind of coil spring for applying an elastic force between the discharge cover 24a and the discharge valve 24b in the axial direction. An O-ring R is inserted onto the inside circumferential surface of one end of the cylinder 4, so that the discharge valve 24a can be closely adhered to one end of the cylinder 4.
An indented loop pipe 28 is installed between one side of the discharge cover 24a and the outlet tube 2b, for guiding the compressed refrigerants to be externally discharged, and preventing vibrations generated by interactions of the cylinder 4, the piston 6 and the linear motor 10 from being applied to the whole closed vessel 2.
Therefore, when the piston 6 is linearly reciprocated inside the cylinder 4, if the pressure of the compression space P is over a predetermined discharge pressure, the valve spring 24c is compressed to open the discharge valve 24b, and the refrigerants are discharged from the compression space P, and then externally discharged along the loop pipe 28 and the outlet tube 2b. 
A refrigerant passage 6a through which the refrigerants supplied from the inlet tube 2a flows is formed at the center of the piston 6. The linear motor 10 is directly connected to one end of the piston 6 adjacent to the inlet tube 2a by a connection member 17, and the suction valve 22 is installed at one end of the piston 6 in the opposite direction to the inlet tube 2a. The piston 6 is elastically supported in the motion direction.
The suction valve 22 is formed in a thin plate shape. The center of the suction valve 22 is partially cut to open or close the refrigerant passage 6a of the piston 6, and one side of the suction valve 22 is fixed to one end of the piston 6a by screws.
Accordingly, when the piston 6 is linearly reciprocated inside the cylinder 4, if the pressure of the compression space P is below a predetermined suction pressure lower than the discharge pressure, the suction valve 22 is opened so that the refrigerants can be sucked into the compression space P, and if the pressure of the compression space P is over the predetermined suction pressure, the refrigerants of the compression space P are compressed in the close state of the suction valve 22.
Especially, the piston 6 is installed to be elastically supported in the motion direction. In detail, a piston flange 6b protruded in the radial direction from one end of the piston 6 adjacent to the inlet tube 2a is elastically supported in the motion direction of the piston 6 by mechanical springs 8a and 8b such as coil springs. The refrigerants included in the compression space P in the opposite direction to the inlet tube 2a are operated as gas spring due to an elastic force, thereby elastically supporting the piston 6.
Here, the mechanical springs 8a and 8b have constant mechanical spring constants Km regardless of the load, and are preferably installed side by side with a support frame 26 fixed to the linear motor 10 and the cylinder 4 in the axial direction from the piston flange 6b. Also, preferably, the mechanical spring 8a supported by the support frame 26 and the mechanical spring 8a installed on the cylinder 4 have the same mechanical spring constant Km.
The linear motor 10 includes an inner stator 12 formed by stacking a plurality of laminations 12a in the circumferential direction, and fixedly installed outside the cylinder 4 by the frame 18, an outer stator 14 formed by stacking a plurality of laminations 14b at the periphery of a coil wound body 14a in the circumferential direction, and installed outside the cylinder 4 by the frame 18 with a predetermined gap from the inner stator 12, and a permanent magnet 16 positioned at the gap between the inner stator 12 and the outer stator 14, and connected to the piston 6 by the connection member 17. Here, the coil wound body 14a can be fixedly installed outside the inner stator 12.
In the linear motor 10, when a current is applied to the coil wound body 14a to generate an electromagnetic force, the permanent magnet 16 is linearly reciprocated by interactions between the electromagnetic force and the permanent magnet 16, and the piston 6 connected to the permanent magnet 16 is linearly reciprocated inside the cylinder 4.
In such a reciprocating compressor, the operation frequency is controlled to be synchronized with the resonance frequency so that the reciprocating compressor can be operated in the resonance state. The resonance frequency is defined as
  f  =            f      m        =                  1                  2          ⁢          π                    ·                                                                  k                m                            +                              k                g                                      m                          .            
Here, km represents the elastic coefficient of the mechanical springs, kg represents the elastic coefficient of the gas springs, and m represents the mass of the linearly reciprocating piston and a member connected thereto. Therefore, the resonance frequency is influenced by the elastic coefficient of the gas springs varied by load, as well as by the elastic coefficient of the mechanical springs, which is a constant. Hence, in the conventional reciprocating compressor, the elastic coefficient of the mechanical springs are set relatively larger than the elastic coefficient of the gas springs to such a degree as to ignore the elastic coefficient of the gas springs in order to easily synchronize the operation frequency with the resonance frequency under low load condition.
The reciprocating compressor used in a cooling system is controlled so as to adjust the flow rate in accordance with a required cooling capacity corresponding to load. The flow rate of the compressor is defined as Q=C×(A×S×f). Here, C represents a proportional constant, A represents a cross-sectional area of the piston, S represents a stroke of the piston which is a reciprocating distance of the piston, and f represents an operation frequency of the piston. Accordingly, in order to adjust the flow rate in accordance with the cooling rapacity corresponding to load, the conventional reciprocating compressor is controlled so that the stroke S of the piston can be increased while approaching to synchronize the operation frequency with the resonance frequency. At this time, when the piston is operated to reach a top dead center operation, in which the head of the piston is consistent with one surface of the cylinder under resonance condition, the gas springs are nonlinearly jumped by a change in input voltage, thereby bringing about an instability phenomenon where the stroke is excessively varied. By using this instability phenomenon, the operation frequency for top dead center operation in the resonance state is determined.
In the above-described conventional reciprocating compressor, the elastic coefficient of the mechanical springs is set larger than the elastic coefficient of the gas springs in order to control the compressor to undergo a resonance operation under low load condition, and, as a result, the shift of the piston is small under overload condition. Hence, even if the piston is operated to reach a top dead center position by synchronizing the operation frequency with the resonance frequency, a sufficient stroke of the piston cannot be generated. Therefore, the conventional reciprocating compressor is controlled to be operated by artificially increasing the stroke of the piston by using asymmetrical logic in place of the top dead center operation that is performed in the resonance state.
FIG. 2 is a view for explaining a method for operating a reciprocating compressor under a load condition according to the conventional art. Referring to FIGS. 1 and 2, when no power is applied to the coil wound body 14a of the linear motor 14 and there is no external force, the distance between one surface of the cylinder 4 constituting the compression space P and the head of the piston 6 (hereinafter, the initial value of the piston) is denoted by Xi.
Under the low load condition, since the ambient temperature is relatively low, the volume of the refrigerant present in the compression space P of the cylinder 4 is relatively small. Thus, there exists no shift of the piston 6 caused by the refrigerant, thereby keeping the initial value Xi of the piston 6 constant.
Accordingly, under the low load condition, a voltage is supplied to have a symmetrical amplitude in order to maintain the stroke of the piston 6 so that the piston 6 can be operated to reach the top dead center (TDC) with respect to the initial value Xi of the head of the piston 6. That is, if the head of the piston 6 is closer to one surface of the cylinder 4 than the initial value Xi, a voltage is supplied which has such an amplitude as to make the head of the piston 6 contact with the one surface of the cylinder 4. On the other hand, if the head of the piston 6 is farther from one surface of the cylinder 4 than the initial value Xi, a voltage is supplied which has an amplitude symmetrical to that when the head of the piston 6 is closer to one surface of the cylinder 4 than the initial value Xi.
Under the overload condition, since the ambient temperature is relatively high, the volume of the refrigerant in the compression space P of the cylinder 4 is relatively small. Thus, there exists a shift of the piston 6 caused by the refrigerant, thereby varying the initial value Xi of the piston 6 by the shift of the piston 6.
Accordingly, under the high load condition, a voltage is supplied to have an asymmetrical amplitude in order to maintain the stroke of the piston 6 so that the piston 6 can be operated to reach the top dead center (TDC) with respect to the initial value Xi of the head of the piston 6. That is, if the head of the piston 6 is closer to one surface of the cylinder 4 than the initial value Xi, a voltage having the same amplitude as that in the low load state. On the other hand, if the head of the piston 6 is farther from one surface of the cylinder 4 than the initial value Xi, a voltage having an amplitude larger than that in the low load state by β(β≧1) is supplied. Of course, it is apparent to those skilled in the art that if a voltage is asymmetrically applied under the overload condition, when the distance between one surface of the cylinder 4 and the head of the piston 6 is farther than the initial value Xi, the movement distance of the piston 6 is increased by β compared to the movement distance of the piston 6 in the low load state. In this way, the flow rate was adjusted by adjusting the stroke of the piston 6 according to load.